1. Field of the Invention
The present invention relates to a method for fabricating a reverse-staggered polycrystalline silicon thin film transistor, and more specifically to a method for fabricating a reverse-staggered polycrystalline silicon thin film transistor wherein a phosphosilicate-spin-on-glass (P-SOG) is used for a gate insulating film.
2. Description of the Related Art
Devices using a polycrystalline silicon thin film are mainly utilized in a variety of appliances including active devices of active matrix liquid crystal displays (AMLCDs), and switching devices and peripheral circuits of electroluminescent (EL) devices.
Since the devices using the polycrystalline silicon thin film are suitable for realization in a high response rate and a large pixel number, they can greatly contribute to obtaining a superior picture quality, large-size screen and excellent color representation of display devices.
Laser beam radiation and solid phase crystallization using a high-temperature heat treatment have been generally used to convert an amorphous silicon film into a polycrystalline silicon film.
In the laser beam radiation, a crystallization process can be performed at a low temperature (i.e., 400° C. or below). Also, a polycrystalline silicon film having a high field-effect mobility can be achieved. However, the laser beam radiation makes it difficult to ensure a uniform crystallization of a sample when the sample has a large area, and requires expensive laser equipment. Thus, there remains a demand for alternative technologies to solve these problems.
Meanwhile, in accordance with the solid phase crystallization using a high-temperature heat treatment, a polycrystalline silicon thin film is fabricated by heating at a high temperature (i.e., 600° C. or above) for a long time. However, the solid phase crystallization requires a high crystallization temperature and a long heating time. Furthermore, polycrystalline silicon grains formed by the solid phase crystallization may have many defects, thereby making it difficult to fabricate devices using the polycrystalline silicon thin film. In addition, due to the high crystallization temperature, it is impossible to use a glass substrate.
Recently, crystallization using a metal medium, (e.g., metal induced crystallization (MIC) and metal induced lateral crystallization (MILC)) has been suggested [G. Liu and S. J. Fonash, Appl. Phys. Lett. 62, 2554 (1993)].
According to the crystallization using a metal medium, a thin film is uniformly formed to produce a polycrystalline silicon thin film transistor having a high field-effect mobility. However, the polycrystalline silicon thin film fabricated by this crystallization may have defects in device structure resulting from a metal contamination, thereby having a limitation on electric properties.
In attempts to solve these problems, various methods have been proposed. For example, metal induced lateral crystallization of amorphous silicon through a silicon nitride cap layer has been suggested [(J. H. Choi, D. Y. Kim, B. K. Choo, W. S. Sohn, and J. Jang. Electrochem. Solid-State Lett, 6, G16 (2003)]. In accordance with this method, a cap layer interposed between an amorphous silicon layer and a metal layer, to control a diffusion of the metal, thereby reducing a metal contamination. Meanwhile, metal-induced crystallization has been employed to obtain a superior polycrystalline silicon thin film while minimizing a metal amount [J. H. Ahn and B. T. Ahn, Journal of The Electrochemical Society, 148, H115 (2001)]. According to this crystallization, a metal ion is deposited in a concentration of 1012 to 1014 cm−2 using an ion injector. The deposited film is subjected to high temperature heating, rapid thermal annealing (RTA) or laser beam radiation, to form a polycrystalline silicon thin film. Then, a mixture of a viscous organic material and a liquid metal is deposited on the thin film by spin coating. The resulting film is subjected to metal-induced crystallization with heating, to obtain a polycrystalline silicon thin film having a leveled surface. However, there has been no technology capable of completely solving a metal contamination, which is a primary problem of the polycrystalline silicon thin film.